The current Code Division Multiple Access (CDMA) wireless communication system envisioned for, inter alia, cellular applications in the United States is defined by TIA/EIA/IS-95A, Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, July 1993. As is the case with any known technology, implementation and adaptation of the known technology into new applications results in new problems. Implementation and adaptation of spread spectrum techniques (specifically, direct sequence CDMA) into wireless communication systems (specifically mobile radiotelephone systems and/or personal communication systems, or PCS) is no different.
One such problem that has originated during the implementation and adaptation of CDMA into a viable wireless communication system is that of the layout of the CDMA wireless communication system in the presence of other previously installed wireless communication systems. A specific example of a previously installed wireless communication system which presents problems to the CDMA system designer is the narrowband frequency-modulated (FM) wireless communication systems, better known as AMPS. To the CDMA system designer, the AMPS system (including locations of AMPS base-stations and the frequencies utilized therein) presents major problems. This is due to the fact that significant interference from an AMPS base-station transmitter (downlink) to a CDMA mobile station occurs. This interference becomes severe when the CDMA mobile station gets close to a same system (operator) non-colocated AMPS base-station or close to other system (operators) AMPS base-station transmitters.
One simple solution to the interference problem mentioned above to mitigate the effects of the AMPS downlink (base-station to mobile-station) interference is to use a step attenuator in the CDMA mobile station receiver. Using a switchable pad in a CDMA mobile station receiver, however, effects the CDMA downlink coverage and other parameters (for example, transmit power) since the CDMA base-station must transmit more power to that particular CDMA mobile station. As CDMA is an interference limited system (i.e., capacity is maximized by minimizing interference), an increase in the CDMA downlink power level for only one CDMA mobile station can effect the operation of the entire CDMA system.
Another challenge to the CDMA system designer is the fact that while planning a CDMA system, the location of potential AMPS interference must be accounted for. This is true because mutual interference between the two types of system (CDMA and AMPS) is expected when the two system coexist in the same geographical area. As such, when placing new CDMA base-stations within a geographical area in which AMPS base-stations are also located, the location of the CDMA base-stations is critical to reduce the amount of mutual interference between the two systems.
Still other challenges exist for the CDMA system designer attempting to implement and adapt the CDMA technology to a wireless communication system. As mentioned above, CDMA is an interference limited system. Consequently, to achieve the maximum of user capacity (and thus system capacity), precise control mechanisms must be utilized in the operation of the CDMA wireless communication system. However, even with precise control, other problems with the control mechanisms (for example, the speed at which control can be administrated) can cause serious problems in the CDMA wireless communication system. Such problems include dropped calls, poor quality during a call, and poor system capacity to name a few.
Thus a need exists for a new method and apparatus for designing and controlling a CDMA wireless communication system.